Electrical Conduction of Conjugated Molecular SAMs Studied by Conductive Atomic Force Microscopy

Abstract

The electrical conduction of self-assembled monolayers (SAMs) made from conjugated molecules was measured using conductive atomic force microscopy (AFM), with a focus on the molecular structural effect on the electrical conduction. For phenylene oligomer SAMs, resistances through the monolayers increased exponentially with increases in molecular length and the decay constants of transconductance β were ca. 0.35 to 0.5 Å-1. Using an insertion technique into insulative alkanethiol SAMs, we successfully obtained single molecular resistance of terphenyl methanethiol at ca. 5.4 × 1010 Ω. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with increases in the applied load up to 15 nN. When two or three methylene spacers were introduced between the sulfur and terphenyl groups in a series of terphenyl derivatized thiols, the monolayer resistances and β values increased extraordinarily. One explanation is that the addition of methylene spacers changed the location of the molecular orbital as a result of MOPAC calculation.

Atomic Force Microscopy Study of the Mechanical and Electrical Properties of Monolayer Films of Molecules with Aromatic End Groups

“…Previous research has shown that π-π intermolecular interactions have a strong impact on the conductivity of organic films [12][13][14][15]19 . Theoretical calculations also indicate that a well-ordered structure should decrease the tunneling resistance across molecules by the better spatial overlap of the HOMO and LUMO molecular orbitals 15 .…”

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

“…2 Studies on various -conjugated molecules have been performed. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The molecules, with delocalized electrons due to sp 2 hybridized carbon atoms, exhibit a smaller energy gap between the highest occupied molecular orbital ͑HOMO͒ and the lowest unoccupied molecular orbital ͑LUMO͒. For -conjugated molecules longer than ϳ3 nm hopping conduction was observed.…”

“…Nevertheless, the ␤-values found for the phenylenes are lower than reported in literature using single molecules as well as SAMs, i.e., ␤ = 0.35-0.61 Å −1 . 3,[5][6][7][8]10 To address the relative low values of ␤, it is interesting to make a comparison with theoretical predictions. Calculations using different formalisms resulted in ␤ = 0.17-0.51 Å −1 .…”

The Diversity of Electron‐Transport Behaviors of Molecular Junctions: Correlation with the Electron‐Transport Pathway

“…), [4,17,18] and carotenoid polyenes (0.22 AE 0.04 À1 ). [19] Small b values (b < 0.1 À1 ) [20][21][22] have also been reported for highly conjugated and low-band-gap systems, such as the alkyne-bridged complex with a b value of 0.04 AE 0.02 À1 , [20] extremely low attenuation of 0.04 AE 0.006 À1 for oligo-zinc porphyrin, [21,22] and 0.06 AE 0.03 À1 for oligoyne series.…”